In centralized postal sorting centers, mail is typically passed through automated sorting systems multiple times. After the last pass through the sorters, the mail must be placed in mail trays and eventually loaded onto trucks by a deadline for dispatching the mail in the trucks to the correct delivery offices. Each truck will typically take mail trays to several different delivery offices, and each of the delivery offices is then responsible for taking the mail to particular final destinations along multiple different mail routes.
Typically, 20 to 40 pieces of various types of sorting equipment are used within a centralized postal sorting center. Often, the mail destined for a single delivery office might be sorted on several types of sorters in different locations throughout the sorting center. After the last pass on each of the sorters, the trays of sorted mail must then themselves be sorted, in order to ensure that all the mail destined for the same destination is loaded in an intelligent order onto the trucks going to that destination.
The average sorting center in the United States Postal Service (USPS) system sorts the mail for about 713 routes, and delivers it to 35 delivery offices, each of which have an average of 20 routes of mail to be delivered. Typically, 60,000 trays of sorted mail must all be sorted and put on the correct trucks at the average sorting center. Trays of mail from various sorter systems (i.e. letter sorters and flats sorters, as well as mail that is manually sorted such as non-machineable mail and newspapers) must be collected together in the same place before they are loaded onto the correct trucks, and/or while they are loaded onto the correct trucks.
Most sorting centers have invested in substantial equipment to transport, store, and retrieve the trays of mail in support of the sorting operations. But often, the final operation of sorting the trays of mail to get them all on the right trucks is a manual operation. In average-sized sorting centers, dozens of workers are required for several hours to sort the trays of mail and get them onto the right trucks on time.
At the most advanced postal services (i.e. “posts”) in the world, typically 20 to 40 workers spend several hours sorting the trays for dispatch manually. In Denmark, an automated tray sorting system has been installed to queue up the trays in front of the correct trucks. That Danish system includes miles of transports, switching networks, and tray label readers to create queues of trays with a common destination. Such a system accepts loaded trays from multiple sorters in the sorting center, transports the trays to a dispatch area, and moves each tray through a series of switches down multiple sidings leading to truck loading areas. Only a few of these systems have been installed around the world because the expense of the automated tray sorting equipment is prohibitive, and it takes many years to pay for itself in labor savings.
Occasionally a tray of mail is loaded onto the wrong truck, so it is sent to the wrong delivery office. Because the deadlines are tight, often such errors are not discovered in time to recover, and it can therefore be very difficult to get the mail to the right place and delivered on the same day as the error was made. So, the service performance of the post is negatively affected by such occasional errors.
What is needed is a way to deliver the mail trays from the sorters to the trucks in exactly the order they are to be loaded onto the trucks. This would have the benefit of reducing the labor hours of the tray sorting staff, as well as reducing the errors of loading trays onto the wrong trucks. And, such a system would have the same benefits as automated tray sorting equipment, but without the prohibitive expense.
Examples of such a clamp-based system can be found in International Application WO 2006/063204 filed 7 Dec. 2005 titled “System and Method for Full Escort Mixed Mail Sorter Using Clamps” and can also be found in U.S. Provisional Application 11/519,630 filed 12 Sep. 2006 titled “Sorter, Method, and Software Product for a Two-Step and One-Pass Sorting Algorithm,” which are both incorporated herein by reference in their entirety. The concepts of macro-sorting are described, for example, in U.S. Provisional Application No. 60/669,340 filed 5 Apr. 2005, titled “Macro Sorting System and Method” which also is incorporated herein by reference in its entirety.